1. Field of the Invention
The present invention relates to a superconducting switch, a superconducting magnet and an MRI.
2. Description of the Related Art
When a superconducting loop circuit is formed of a superconducting coil and a superconducting switch, a superconductor used for the superconducting switch is generally selected from superconductors used for the superconducting coil. Although there are an immersion cooling method and a conduction cooling method as cooling means of a superconductor, the same cooling method is usually adopted for the superconducting coil and the superconducting switch constituting a certain superconducting circuit.
The superconducting switch generally performs switching by heating the superconductor by a heater. When the superconductor is cooled to the critical temperature or lower, the resistance becomes zero (ON state), and when heated to the critical temperature or higher, the superconductor becomes a normal conductor and a resistance is generated (OFF state).
At the time of excitation of a superconducting magnet, the superconducting switch is placed in the OFF state, and most of the current supplied from an excitation power supply is made to flow through the superconducting magnet. In order to speed up ON/OFF switching of the superconducting switch or in order to suppress a refrigerant evaporation amount during the switching, a temperature difference between the temperature of the superconducting switch in the ON state and the critical temperature is preferably as small as possible. On the other hand, if the set temperature of the superconducting switch in the ON state is close to the critical temperature of the superconductor, when disturbance is applied to the superconducting switch, the temperature of the superconductor approaches the critical temperature and quenching easily occurs. Thus, the stability of the superconducting switch becomes low.
Since a low temperature superconductor such as NbTi is generally cooled by liquid helium, in the superconducting switch using the low temperature superconductor, the temperature in the ON state is the liquid helium temperature (about 4K) and the temperature in the OFF state is approximately the critical temperature (about 9K). In this case, the temperature of the superconductor of the superconducting switch is raised by about 5K by heating using a heater.
Recently, high temperature superconductors are found, and the critical temperature of the superconductor becomes high. For example, when the superconducting switch using the high temperature superconductor with a critical temperature of 90K is used in liquid helium, the temperature of the superconducting switch is required to be raised from 4K to 90K. In order to efficiently perform this heating, heat transfer efficiency between a superconducting film and a heater is required to be enhanced.
For example, patent document 1 (JP-A-2003-142744) discloses a superconducting switch using a high temperature superconducting film, and YBCO and the like are exemplified as the high temperature superconductors.
However, in the patent document 1, since an insulating substrate having a thickness sufficient to keep the strength is included between the superconducting film and a heater, there is a problem that thermal capacity of the insulating substrate existing between the superconducting film and the heater is large.